A special USB cable that connects
the circuit board to your computer. You'll need it if you build the
USB version. $20

Encoders for the two axis:

Optical Encoders:

Ek
usually encourage builders to choose encoders that result in
somewhere between 4000 and 10000 encoder tics/rev, factoring in any gearing
or pulley systems that are used to connect the encoders to the telescope.